The preparation of styrene diene block copolymers (“SBC”) is well known. In a representative synthetic method, an initiator compound is used to start the polymerization of one monomer. The reaction is allowed to proceed until all of the monomer is consumed, resulting in a living homopolymer. To this living homopolymer is added a second monomer that is chemically different from the first. The living end of the first polymer serves as the site for continued polymerization, thereby incorporating the second monomer as a distinct block into the linear polymer. The block copolymer so grown is living until terminated. Termination converts the living end of the block copolymer into a non-propagating species, thereby rendering the polymer non-reactive towards a monomer or coupling agent. A polymer so terminated is commonly referred to as a diblock copolymer. If the polymer is not terminated the living block copolymers can be reacted with additional monomer to form a sequential linear block copolymer. Alternatively, the living block copolymer can be contacted with multifunctional agents commonly referred to as coupling agents. Coupling two of the living ends together results in a linear triblock copolymer having twice the molecular weight of the starting, living, diblock copolymer. Coupling more than two of the living ends together results in a radial block copolymer architecture having at least three arms.
One of the first patents on linear ABA block copolymers made with styrene and butadiene is U.S. Pat. No. 3,149,182. These polymers in turn could be hydrogenated to form more stable block copolymers, such as those described in U.S. Pat. Nos. 3,595,942 and Re. 27,145. Selective hydrogenation to remove the C═C moieties in the polydiene segment of such polymers is critical in preparing block copolymers with good thermal and chemical resistance, particularly resistance to oxidative degradation.
Through the years, there have been many modifications made to such block copolymers to change and improve properties. One such modification has been to sulfonate the block copolymer. One of the first such sulfonated block copolymers is disclosed, for example, in U.S. Pat. No. 3,577,357 to Winkler. The resulting block copolymer was characterized as having the general configuration A-B-(B-A)1-5, wherein each A is a non-elastomeric sulfonated monovinyl arene polymer block and each B is a substantially saturated elastomeric alpha-olefin polymer block, said block copolymer being sulfonated to an extent sufficient to provide at least 1% by weight of sulfur in the total polymer and up to one sulfonated constituent for each monovinyl arene unit. The sulfonated polymers could be used as such, or could be used in the form of their acid, alkali metal salt, ammonium salt or amine salt. In the Winkler patent, a polystyrene-hydrogenated polyisoprene-polystyrene triblock copolymer was treated with a sulfonating agent comprising sulfur trioxide/triethyl phosphate in 1,2-dichloroethane. Such block copolymers exhibited water absorption characteristics that might be useful in water purification membranes and the like.
The sulfonation of unsaturated styrene-diene block copolymers is disclosed in O'Neill et al, U.S. Pat. No. 3,642,953. Polystyrene-polyisoprene-polystyrene was sulfonated using chloro-sulfonic acid in diethyl ether. Since the sulfonic acid functionality incorporated into the polymer promotes oxidation and the residual C═C sites left in the polymer backbone are prone to oxidation, these polymers had limited utility. As stated in column 3, line 38, of this patent: “The unsaturated block copolymer sulfonic acids obtained by this process are subject to rapid oxidative degradation in air, therefore, they must be handled under anaerobic conditions and/or stabilized with anti-oxidants until they have been cast from solution into their final form and converted to the more stable salt by neutralization or ion-exchange.” The sulfonated, unsaturated block polymers prepared in the experiments outlined in the Examples of the O'Neill et al patent were cast as produced into thin films. The films exhibited excessive swelling (up to 1600% wt water uptake) and were weak. While the cast films could be stabilized by treatment with an excess of base and their properties did improve somewhat on neutralization (still only 300 to 500 psi of tensile strength); the films in the sulfonate salt form were now insoluble and could not be reshaped. Similarly, Makowski et al, U.S. Pat. No. 3,870,841 includes examples of sulfonation of a t-butylstyrene/isoprene random copolymer. As these sulfonated polymers have C═C sites in their backbone, they are not expected to be oxidatively stable in the sulfonic acid form either. Such polymers were used for applications requiring limited flexibility, and are not expected to have acceptable overall physical properties. Another sulfonated styrene/butadiene copolymer is disclosed in U.S. Pat. No. 6,110,616, Sheikh-Ali et al, where an SBR-type random copolymer is sulfonated.
Another route to make sulfonated block copolymers is disclosed in Balas et al, U.S. Pat. No. 5,239,010, where an acyl sulfate is reacted with a selectively hydrogenated block copolymer composed of at least one conjugated diene block and one alkenyl arene block. After hydrogenation, the block copolymer is modified by attaching sulfonic acid functional groups primarily in the alkenyl arene blocks (A blocks). The mechanical properties may be varied and controlled by varying the degree of functionalization (amount of sulfonation) and the degree of neutralization of the sulfonic acid groups to metal sulfonated salts.
In Pottick et al, U.S. Pat. No. 5,516,831, a blend of an aliphatic hydrocarbon oil and a functionalized, selectively hydrogenated block copolymer to which has been grafted sulfonic functional groups is disclosed. In the block copolymer of Pottick et al, substantially all of the sulfonic functional groups are grafted to the block copolymer on the alkenyl arene polymer block A, as opposed to the substantially completely, hydrogenated conjugated diene block copolymer B. Neutralization of the acid groups to a metal salt was preferred to prepare oil extended blends that retained substantial amounts of non-extended mechanical properties. The block copolymer blends were used for adhesives and sealants, as modifiers for lubricants, fuels and the like.
Recently there has been more attention given to the use of sulfonated block copolymers for fuel cells. For example, Ehrenberg et al, U.S. Pat. No. 5,468,574, discloses the use of a membrane comprising a graft copolymer of sulfonated styrene and butadiene. In the examples, an SEBS block copolymer (i.e., a selectively hydrogenated styrene/butadiene/styrene triblock copolymer) was sulfonated with sulfur trioxide to an extent of at least 25 mol percent basis the number of styrene units in the block copolymer. As shown in the patent, the sulfonic acid groups are all attached to the styrene units. The deleterious effects of water induced swelling in such membranes are discussed in the article by J. Won et al, titled “Fixation of Nanosized Proton Transport Channels in Membranes”, Macromolecules, 2003, 36, 3228-3234 (Apr. 8, 2003). As disclosed in the Macromolecules article, a membrane was prepared by solvent casting a sample (from Aldrich) of a sulfonated (45 mol % basis styrene content) SEBS (Mw about 80,000, 28% w styrene) polymer onto glass. The membrane was immersed in water and found to absorb over 70% of its dry weight in water as a consequence of swelling. The rate of methanol transport through the water-swollen membrane was then tested and found to be undesirably high. This is not a preferred result for direct methanol fuel cell (DMFC) applications where segregation of methanol to only one compartment of the cell is essential for the device to generate electrical power. For these applications, “methanol crossover needs to be reduced, while maintaining proton conductivity and mechanical strength, to improve fuel cell performance.” This problem was overcome to a certain extent, as described in the report by J. Won et. al, by first casting a film of a styrene-diene block copolymer, radiation crosslinking the film (cSBS), and then sulfonating the pre-formed article. While crosslinking the block copolymer prior to sulfonation overcame the problem of excessive swelling that was observed when an S-E/B-S polymer that was selectively sulfonated in the outer blocks was used to form a membrane, crosslinking technology is limited in its utility to thin, transparent articles that are readily penetrated by the radiation source. In addition, sulfonation of the crosslinked article is time consuming and uses an excess of dichloroethane (DCE). As reported by J. Won et. al, “The cSBS film was swollen in an excess amount of DCE overnight. The solution was heated to 50° C. and purged with nitrogen for 30 min. Then the acetyl sulfate solution (produced with the procedure described above) was added.” “The solution was stirred for 4 h at this temperature, and then the reaction was terminated by the addition of 2-propanol, resulting in a sulfonated SBS cross-linked membrane (scSBS).” Cleaning up the sulfonated article was also problematic. “The membrane was washed in boiling water and many times with cold water. The complete removal of residual acid from the final product after sulfonation is important since it can interfere with the properties of the final product.”
Still another type of block copolymer that has been sulfonated in the past is selectively hydrogenated styrene/butadiene block copolymers that have a controlled distribution interior block containing both styrene and butadiene, as opposed to the normal block copolymers that just contain butadiene in the interior block. Such block copolymers are disclosed in Published U.S. Patent Application Nos. 2003/0176582 and 2005/0137349, as well as PCT Published Application WO 2005/03812.
In the sulfonated block copolymers disclosed above, invariably the outer (hard) blocks are sulfonated due to the presence of styrene in the outer blocks. This means that upon exposure to water, hydration of the hard domains in the material will result in plasticization of those domains and significant softening. This softening of the hard domains results in a marked decrease in the mechanical integrity of membranes prepared from these block copolymers. Thus, there is a risk that when exposed to water any structure supported by these prior art sulfonated block copolymers will not have sufficient strength to maintain its shape. Hence, there are limits to how to use such a block copolymer and limits on its end use applications.
Other prior art sulfonated polymers are taught where the end blocks and interior blocks do not include hydrogenated dienes. U.S. Pat. No. 4,505,827 to Rose et al relates to a “water-dispersible” BAB triblock copolymer wherein the B blocks are hydrophobic blocks such as alkyl or sulfonated poly(t-butyl styrene) and the A blocks are hydrophilic blocks such as sulfonated poly(vinyl toluene). A key aspect of the polymers disclosed in Rose et al is that they must be “water dispersible”, since the uses contemplated for the polymer are for drilling muds or for viscosity modification. Rose et al states at column 3, lines 51 to 52 that the polymer “exhibits hydrophobe association capabilities when dispersed in an aqueous medium.” Rose et al. goes on to state in lines 53 to 56 that “[F]or purposes of the invention, such a polymer is one which, when mixed with water, the resulting mixture is transparent or translucent, and not milky white as in the case of a dispersion of a water-insoluble polymer.” The polymer of Rose et al. is water-dispersible since the t-butyl styrene blocks are not large—typically the block copolymer will have less than 20 mole percent of B blocks, preferably from about 0.1 to about 2 mol percent. In addition, the B end blocks will likely contain a significant amount of sulfonation.
U.S. Pat. No. 4,492,785 to Valint et al. relates to “water soluble block polymers” which are viscosification agents for water. These water-soluble block copolymers are either diblock polymers of t-butyl styrene/metal styrene sulfonate or triblock polymers of t-butyl styrene/metal styrene sulfonate/t-butyl styrene. It appears from the structures and properties given that the interior block styrene is 100% sulfonated. This will result in the polymer being water-soluble. Further, in the structures given, each of the end blocks will comprise 0.25 to 7.5 mol percent of the polymer. With such a large interior block that is fully sulfonated, and has relatively small end blocks, the polymers will invariably be water-soluble.
None of the prior art references noted above disclose sulfonated polymers based on styrene and/or t-butyl styrene that are in a solid state in the presence of water and have both high water transport properties and sufficient wet strength. Accordingly, what is needed is a semi-permeable membrane with high water transport properties that maintains sufficient wet strength for a wide variety of applications.